This is the single most common expensive mistake I see on S corporation returns, and it is almost never caught by the preparer who made it. The situation A construction S corporation. One shareholder. The company had a bad year and lost about $180,000. The owner had put $20,000 into the company when he started it and nothing since. The bank had lent the company $200,000. The bank would not lend to the company alone, so the owner personally guaranteed the loan. He signed. His house was on the line. In his mind, and in his previous preparer's mind, he was on the hook for $200,000, so he had $200,000 at risk, so he could deduct the loss. The prior return deducted the full $180,000 on his 1040. What the law actually says A shareholder gets basis two ways. Money or property he puts into the corporation for stock, and money he lends directly to the corporation. That is the list. A guarantee is not on the list. Guaranteeing a corporate debt gives the shareholder no basis at all, and it does not matter how real the exposure feels. The Fourth Circuit settled this in Estate of Leavitt back in 1989 and the regulations say the same thing today: basis comes from an actual economic outlay, and signing a guarantee is not an outlay. It becomes one only if the guarantee is called and the shareholder actually pays. So his basis was $20,000. Not $200,000. What the number really was - Deductible loss in the current year: $20,000, which is his basis. - Suspended and carried forward: $160,000. - Deducted on the return as filed: $180,000. - Overstated by: $160,000, on a return that had already been accepted. Here is the part that is actually useful The suspended $160,000 is not gone. It carries forward with no expiration and it comes free the moment he has basis again. The fix is structural and it is not complicated. Instead of guaranteeing the company's bank loan, the shareholder borrows from the bank personally and then lends that money to the corporation. Same bank, same money, same personal exposure, and now it is a direct shareholder loan, which is debt basis. The economics barely change. The tax result changes completely.